This invention relates to a method and equipment for weld overlay of metal tubing. More specifically, the invention relates to a system which employs a welding process to overlay conventional metal tubing in a single pass.
Boilers are used in the power generation, paper, steel and waste-to-energy industries, and these industries have an interest in finding means to protect and extend the operating cycles of their boilers. The concerns of boiler operating cycles arise from the fuel used by the boiler, methods of combustion, and/or impact of environmental regulations. Boilers generally use tubing, and such tubing may have structural characteristics and must be corrosion resistant as well, in view of the high temperature and corrosive conditions within boilers.
In certain types of boilers, such as coal-fired, black liquor recovery and waste-to-energy boilers, there is a particular need for corrosion and/or erosion resistance. In such instances, a solution is normally found in applying overlay to tubing used in the boilers with an alloy having desired corrosion and erosion protection.
When tubing is weld overlaid, the weld penetration of the tubing needs to be carefully controlled to ensure good fusion between a weld overlay bead and tube stock, as well as between each weld bead. The exterior surfaces of the weld overlay need to be smooth, and the thickness of the weld overlay needs to be uniform. Conventional welding processes may produce excessive weld penetration, with dilution of the base metal into the weld metal in excess of 40%. Weld overlays with this high dilution are not acceptable.
In conventional application of weld overlay, if the voltage and current of the weld head are controlled to ultimately provide a smooth exterior surface on the weld overlays, then weld penetration of the tube stock may tend to be excessive and uneven, and this can result in excessive dilution.
On the other hand, if the voltage and current are controlled to provide optimum weld penetration, then the exterior surface of the weld overlay may tend to be rough and uneven. A rough surface finish provides greater chance for corrosion media to deposit and start localized corrosion sites in the overlays. This can also make tubes more difficult to bend, which can result in an uneven distribution of the stresses during bending. A rough and uneven weld overlay surface can also affect the thickness and resulting dimensions of the tube.
Certain weld overlay methods have been patented. For example, U.S. Pat. No. 6,013,890, issued to Hulsizer, discloses a dual weld pass overlay method and apparatus which uses a first weld head to apply a bead of weld overlay material onto a tube, and thereby create a heat-affected zone in the tube. A second weld head is then used to heat the material within the heat-affected zone to xe2x80x9ca temperature higher than its tempering temperature but lower than its Ae1 temperature.xe2x80x9d U.S. Pat. No. 6,204,477 B1, issued to Lai, also discloses a weld overlay method.
However, a need still exists for a weld overlay process which can provide uniform weld penetration on tube stock, uniform fusion, a smooth and even exterior surface, and also, which can control the thickness of the overlay and the dimensions of the pipe.
It is, therefore, the principal object of this invention to provide a method and apparatus for performing a single pass weld overlay.
Another object of the present invention is to provide a weld overlay system.
Another object of the present invention is to provide a weld overlay system using a gas tungsten arc welding head for applying a weld overlay bead to a tube.
Still another object of the present invention is to provide a weld overlay system using a gas tungsten arc welding head with high frequency wave pulse current to apply a weld overlay bead onto a tube.
Yet another object of the present invention is to provide a weld overlay system wherein heat is applied to weld material prior to being applied to a tube.
A still further object of the present invention is to provide a weld overlay system using a gas tungsten arc welding process which provides reduced dilution of the base metal into the weld metal of the tube.
Another object of the present invention is to provide a weld overlay system for minimizing weld penetration into the tubing, providing proper uniform fusion, and for providing a smooth and uniform exterior surface of the weld overlay.
Yet a further object of the present invention is to provide a weld overlay system offering improved control of the thickness of the overlay applied to a tube.
The present invention includes a process for applying a weld overlay to a tube with a single pass of a weld head. In a preferred embodiment, a gas tungsten arc welding process with high frequency wave pulse current is used to apply heat to a tube. Overlay material in the form of weld wire is pre-heated and mechanically fed into the resultant weld pool. The wire is resistance heated to a temperature lower than its melting point just before the wire contacts the molten weld pool.
The pre-heating of the wire, together with use of the high frequency wave pulse gas tungsten arc welding head, are significant features of the present invention. The control of these parameters produces weld beads with improved uniformity of weld penetration and fusion. Also, the present invention provides for improvements in the control of dilution, composition of the weld overlay, and in the smoothness and evenness of the exterior surface finish. The present invention provides relatively accurate control of thickness of the overlay and, consequently, of the final dimensions of the overlaid tube.
Preferably, the tube is positioned on rollers to support the tube""s weight, and the tube is rotated with respect to the weld head while the weld head moves along the longitudinal axis of the tube.